docs: adding PD manual (mkdocs) (#18833)

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# Overview

Adding files for an updated PD manual, built with mkdocs. This PR is a
mix of 1) getting the text and images into mkdocs and 2) changes to the
manual for REV2 and to better fit the mkdocs format.

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## Test Plan and Hands on Testing

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## Changelog

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adds new markdown files and images to a docs folder. 
changes to the PD manual include new text for PD 8.5.0 (liquid classes,
PD > Python, other small changes), and additional images, headers, etc.
plus some text tweaking to better fit the mkdocs format.

TODO:
- remove lightbox from yml? or enable only on specific pages? 

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Co-authored-by: Edward Cormany <[email protected]>

Author: emilyburghardt